It won't effect the number port but obviously you won't be able to use the sim until you top up.
I have no affiliation whatsoever with O2 or any subsidiary companies. Comments posted are entirely of my own opinion. This is not Customer Service so we are unable to help with account specific issues.
Currently using: Pixel 7a (O2 & Lyca), One Plus 6 (Sfr), iPhone 12 Pro Max (Vodafone) >>>>>>>>>>>>>>>>